This invention relates to asynchronous channel frequency scanning associated with a communication device. The invention is particularly suitable for, but not limited to, channel frequency scanning of a channel frequency scan list of selective call receivers.
Portable communication devices are becoming widely accepted as a form of communication. One type of communication device is known as a selective call receiver, often referred to as a pager, which receives paged information transmitted from one or more transmitters in a system. In this specification, selective call receivers will be used by way of example, however the invention is not necessarily limited to selective call receivers. Such selective call receivers are usually carried by users for receiving paged messages when they are away from their office or home telephone.
As communication receiver users travel more, it is preferable to have a communication receiver, known as a roaming communication receiver, which can operate or roam in one or more selective call systems while still being able to receive relevant messages. Such systems provide transmission of selective call messages from a message originator who contacts a selective call terminal and provides the contents of the message and information identifying the intended selective call receiver user. The selective call terminal encodes the message into one of several known protocols, such as the POCSAG signalling code or the FLEX(trademark) high speed paging protocol. In addition, the terminal appends an address assigned to the communication receiver to the message. The address and message are then modulated onto a selective call signal and transmitted from the selective call terminal.
In systems that employ simulcast techniques as used in the FLEX(trademark) protocol and as mentioned, for example, in U.S. Pat. Nos. 4,696,051 and 4,696,052, a selective call receiver identifies and monitors a valid channel in order to receive messages. In the remainder of this specification channel includes a message transmission frequency and/or System Identification Information (SII). When considering the FLEX(trademark) protocol, this System Identification Information (SII) includes either or both a Network Identifier (NID) or Simulcast System Identifier (SSID). However, other identifiers and other protocols may provide such System Identification Information (SII).
Roaming communication receivers generally have a frequency scan list of possible valid channel frequencies that are periodically scanned. However, depending upon protocol used, a roaming communication receiver may need to be synchronized to a valid channel. Further, such roaming communication receivers must identify that a channel frequency is associated with a valid channel (i.e., the channel is transmitting messages in one or more specific protocols usable by the receiver).
To identify a valid channel a communication receiver has a scan list of channel frequencies store in nonvolatile memory (usually known as a codeplug). These frequencies are usually continuously used to determine which channels are available to the communication receiver. In this regard, the receiver is usually required to frequently scan all relevant channel frequencies in the scan list during a detection portion (xe2x80x9cpower upxe2x80x9d portion) of a scan interval. If any channels are detected during scanning they are then usually processed to determine if they are transmitting messages in one or more specific protocols.
In asynchronous scanning, the communication receiver and channels are not inherently synchronized. It is therefore usually necessary, for example the Flex(trademark) protocol, to detect one or more mandatory frames transmitted in the channels in order for the communication receiver to adequately receive messages transmitted on the channels. Unfortunately, in order to detect these mandatory frames scanning and processing of channels (i.e., baud detection) is required which causes power drain on the power supply of the communication receiver, which for a selective call receiver is usually a type AA or AAA battery. Accordingly, if there are numerous channels (typically between 5 to 15) this can result a relatively rapid battery drain which may be unacceptable.
It is an aim of the invention to overcome or alleviate at least one of the problems associated with scanning mandatory frames transmitted on channels for use by a communication receiver.
According to one aspect of the invention there is provided a communication device comprising:
receiver means for receiving coded messages transmitted on one or more channels;
processing means, in communication with said receiver means, for processing one or more of said messages;
channel control means coupled to both said processing means and said receiver means, said channel control means being adapted to control said receiver means to thereby control reception of said channels; and
a memory means storing a group of channel frequencies, said memory means being coupled to said processing means;
wherein said processing means is adapted, in combination with said control means, to control said receiver means to scan at least some of said channels during baud detect periods of a protocol cycle associated with at least one of said channels; and
wherein said processing means cyclically offsets a start of said baud detect periods after completion of a protocol cycle.
Preferably, processing means may be adapted to cyclically offset said start of said baud detect periods by an offset period, said offset period being relative to a start time of a sample interval length.
Suitably, said processing means can be adapted such that said sample interval length is divisible, without a remainder, by said offset period.
Preferably, said communication device may be a selective call receiver.
According to another aspect of the invention there is provided a method for detecting mandatory frames of a channel protocol received by a communication receiver, the method comprising the steps of:
scanning channels during baud detect periods of a protocol cycle;
processing information received during said step of scanning; and
cyclically offsetting a start of said baud detect periods after completion of a protocol cycle.
Preferably, said method may be further characterized by said step of cyclically offsetting said start of said baud detect periods by an offset period that is relative to a start time of a sample interval length.
Suitably, said method may be further characterized by said sample interval length being divisible, without a remainder, by said offset period.
Preferably, for a given number of mandatory frames w having a frame time ft, and for pre-determined number of cycles y equal to the number of times said sample interval length is divisible by said offset value, said sample interval length is no greater than:
y.((wxe2x88x921).ftxe2x88x92duration of one baud detect period).
Suitably, at least some of said channels scanned during said baud detect periods may be removed or replaced with other channels.
Preferably, said method may be further characterized by said communication device being a selective call receiver.